How to convert an HP 66311B power supply from 120 VAC to 230 VAC

Published in July 2013

This article describes how I converted an HP 66311B "Mobile Communications DC Source" from 120 VAC to 230 VAC. The conversion is quite straightforward as it only involves reconnecting some leads on the primary of the mains transformer inside the unit. It should of course not be attempted by anyone who is not qualified and the mains cable must of course be unplugged before doing the conversion.

1. Disassembly

Unplug the mains cable.

Remove the screws on the bottom (near the back) and at the sides of the back panel. The innards should now be able to slide out through the front side of the box.

2. Reconnection of transformer

Turn the power switch on and measure the resistance between the two power pins in the mains input connector. It should be around 2.7 Ω or so for a unit that is configured for 120 VAC operation.

Identify the big mains transformer close to the front panel.

The attaching cables should currently be connected as below. (I number the pins with 1 starting at the left and 7 at the right.)

Transformer primary side connection in 120 VAC configuration. There are jumpers between pins 1 and 3 and between pins 2 and 7. The mains leads connect to pins 4 and 6. The resistance at the mains input is about 2.7 Ω.

• Remove the long jumper (from pins 2 and 7). It is not needed anymore.
• Remove the left mains lead (from pin 4).
• Remove the left end of the short jumper (from pin 1).
• Reconnect the left end of the short jumper to pin 2.
• Reconnect the left mains lead to pin 1.
• Verify that the resistance at the mains connector is now about 9 Ω

After these steps are performed, the transformer should be connected as shown below.

Transformer primary side connection in 230 VAC configuration (after modification). There is a jumper between pins 2 and 3 and mains connects to pins 1 and 6. The resistance at the mains input when the power switch is on should be around 9 Ω. (Click to enlarge.)

3. Re-assembly

Put the unit back together again.

Using a felt pen or similar, change the marking on the back side to show that it now works off of 230 VAC.